I have never seen an 485 optoisolated at the output line...
Just because you'll have a lot of problems with line impedance.
Use optocouplers isolation ( you need three ) and a separate power supply
for line driver/receiver.
However, if it works with 1:1 transformers and at least 30 devices on one
1200m EIA485 line, please let me know ! It should be interesting !

If you mean by basic flyback, an isolated power supply, then computing the
transformer means only using the Faradys law at a known F and B

> Hi All,
>
> I want to build a BASIC FLYBACK to isolate the serial communication on a PCB
> (485). I want to supply the isolated section by a transformer. Do you any
> idea for the transformer...
>
> I was using a MAX243 with a coilcraft transformer, but it is too
> expensive...
>
> Thanks a lot,
>
>
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At 06:19 PM 12/4/02 -0500, you wrote:
>Hi All,
>
>I want to build a BASIC FLYBACK to isolate the serial communication on a PCB
>(485). I want to supply the isolated section by a transformer. Do you any
>idea for the transformer...

Why exactly would you select the flyback topology for this application?

>I was using a MAX243 with a coilcraft transformer, but it is too
>expensive...

Do you mean MAX253? It's convenient, but can be replaced with some
standard parts at a component cost savings (not to mention availability).

Usually you design the transformer along with the rest of the circuit for
DC-DC converters. See Pressman, Billings etc. for the gory details.